By producing exopolysaccharides, both S. thermophilus and Lb. bulgaricus contribute to the viscosity and the smooth texture of fermented products like yogurt. They stabilize the gel and decrease its tendency to synerise (78). These bacteria produce rather low amounts of exopolysaccharides with a wide variety of chemical structures. Glucose, galactose and rhamnose are its main monomers, and the composition might be affected by the fermentation conditions (79,80). The chemical structure of the polymer apparently has an effect on the rheological properties, because the use of different starters results in differences in its microscopic structure and viscosity (81). The underlying mechanism of this effect, however, is not well understood. The emerging knowledge on the genetics of exopolysaccharide biosynthesis by L. lactis and S. thermophilus strains (Fig. 5) (82,83) will offer opportunities to modify the structure of the polymer and to regulate the amount synthesised.

Regulation Glycosyltransierasea Polymerization mm

Figure 5 Genetic map of the genes involved in EPS production in L. lactis. (From Ref. 83.)

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